Conventional proximity sensors require a time-varying magnetic field which is usually supplied by an oscillator circuit in conjunction with a sensor coil. The change in the magnetic field due to a nearby magnetic object or the change in the losses of the coil due to eddy currents in a nearby metallic object are measured by the oscillator circuit. Of necessity, the proximity sensor only measures the present of objects in the immediate vicinity of the sensor coil. In order to acquire information over an extended region of space, either the sensor must be moved or a multiplicity of sensors must be provided.
Magnetoacoustic devices have been known in the art for some time to provide various sensing and data storage functions. The textbook, Physical Acoustics, Volume IA, edited by Mason, summarizes a considerable body of knowledge reating to the theory and application of these devices. U.S. Pat. Nos. 4,298,861 and 3,898,555 to Tellerman use a magnetostrictive wire in conjunction with a movable magnet to indicate the position of a piston in a cylinder or of depressed keys on a keyboard. Others have used the propagating magnetoacoustic wave to sense the position of a movable coil.
These sensors have a number of drawbacks. For example, the concept of Tellerman requires the use of magnets to provide the proper sensing action. Sensing the voltage in a movable pickup coil also limits the possible applications. Furthermore, none of the inventions previously set forth provide for the sensing of arbitrary ferrous and non-ferrous or ferrite objects not having a permanent magnetic field.